1. Field of the Invention
The present invention relates to permanent magnet assemblies for false tooth stabilization which can be used for holding a false tooth by magnetic attraction provided by a permanent magnet.
2. Description of the Related Art
Various attempts have been made to use magnetic attraction acting between a permanent magnet and a soft magnetic alloy for fixing a false tooth into the oral cavity. For example, techniques representing such attempts are described in "The Application of Rare Earth Magnetic Retention to Osseointegrated Implants, Oral Maxillo-Facial Implant", Vol. 1 (1987), No. 2, pages 77 to 89, by T. R. Jackson.
In order to place the above-mentioned type of false tooth, several conditions are required. For example, one of the conditions is that the permanent magnet be completely hermetically sealed in a case formed of a material which is harmless to a human body. Another condition is that the permanent magnet have a small magnetic flux leakage to the outside.
An example of a conventionally used permanent magnet assembly which has such a structure is shown in FIGS. 5(a) and 5(b). FIG. 5(a) is a perspective view of the permanent magnet assembly, showing the outer appearance thereof and FIG. 5(b) is a diametric cross section of the permanent magnet assembly. As shown in FIGS. 5(a) and 5(b), a permanent magnet 1, which is formed of a rare earth cobalt material or the like, in a cylindrical shape, and which is also magnetized in an axial direction shown by an arrow in FIG. 5(b), is stored in a case 2 formed of stainless steel, having magnetic and high corrosion resistance and also which has a U-shaped cross section to define an overall cup-shaped configuration. The case 2 is covered and hermetically sealed by a thin disc cover plate 3 which is formed of non-magnetic stainless steel having a high corrosion resistance. These components are then bonded together by use of an adhesive agent 4, thereby to provide a permanent magnet assembly 10 which has a closed structure.
FIG. 6 is a cross sectional view of main portions of false tooth fixing means which uses a conventional permanent magnet assembly. In FIG. 6, a root surface member 7, which is formed of a soft magnetic alloy with a T-shaped longitudinal section, is embedded in an alveolus 9. A dental plate 8 is shown in which a permanent magnet assembly 10 shown FIG. 5 is to be embedded in such a manner that a portion thereof located on the side of the cover plate 3 is opposed to the root surface member 7. This structure allows magnetic attraction to act between the permanent magnet assembly 10 and root surface member 7. The magnetic attraction pulls the dental plate 8 against the alveolus 9, so that the dental plate 8 can be stabilized in the mouth.
In this case, the magnetic flux from the permanent magnet 1 passes through a magnetic path including the root surface member 7, the case 2, and the permanent magnet 1, and, therefore, magnetic flux leakage to the outside is very small. Also, because the case 2 and cover plate 3 are both made of stainless steel, they have sufficient corrosion resistance and wear resistance against the biting force of the false tooth 11. Further, the adhesive agent 4 is also said to have sufficient adhesive strength, air tightness when it is used in the oral cavity, and chemical stability.
However, the conventional permanent magnet assembly 10 constructed in the above-mentioned manner has several problems to be solved. For example, although the adhesive agent 4 used to bond the respective components of the assembly 10 together is said to have a sufficient adhesive property, because it is formed of an organic compound, when used for a long time, the adhesive agent 4 is not likely to maintain its chemical stability nor its adhesive strength. For this reason, a permanent magnet assembly of a closed type having a higher reliability is required. Also, while the magnetic circuit of the above-mentioned conventional permanent magnet assembly 10 minimizes leakage of magnetic flux, it suffers from a reduction in the magnetic attracting force that is a most important property. Therefore, a permanent magnet assembly which has a greater attracting force and is thus able to hold a false tooth in a stable manner is needed.
In order to solve the above-mentioned problems, the present applicants have previously filed patent applications (Japanese Patent Application Nos. 2-157285 and 3-141691 of Heisei), each of which relates to a permanent magnet assembly for false tooth stabilization, comprising a permanent magnet, a case formed of a corrosion resistant and magnetic material having a recessed portion for storing the permanent magnet, and a seal plate formed of a corrosion resistant material for covering the case to prevent the permanent magnet from being exposed from an opening of the case recess. The seal plate is formed with a seal edge member of a corrosion resistant and non-magnetic material and a central seal plate member formed of a corrosion resistant and soft magnetic material. The central seal plate member has an outer peripheral shape substantially the same as the inner peripheral shape of the seal edge member. Also, the case and seal plate are welded to each other in the respective abutting portions thereof.
According to the above invention, the permanent magnet 1 can be closed completely without exposing the adhesive agent 4 as shown in FIG. 5 to the outer peripheral surface of the permanent magnet assembly 10 or without using any adhesive agent 4 and, for this reason, reliability in the closure is enhanced. Also, since a magnetic air gap between the permanent magnet 1 and root surface member 7 shown in FIG. 6 is minimized, the attraction can be increased and the degree of resistance to wear can be improved. However, in recent years, a higher degree of performance has been required of the permanent magnet assembly 10 of this type as well as stronger magnetic attraction.
Also, according to the permanent magnet assembly for false tooth stabilization proposed by the above-mentioned Japanese Patent Applications Nos. 2-157285 of Heisei and 3-141691 of Heisei, the abutting portions of the case and seal plate as well as the abutting portions of the seal edge member and central seal plate member are welded to each other by a laser. However, when the case and seal plate are laser welded to each other in the abutting portions thereof, they are distorted due to heat generated in welding and thus subject to air gaps or undulations between the abutting portions of the seal edge portion and seal plate member. This causes unwanted dimensional variation of the permanent magnet assembly.